1. Field of the Invention
The invention relates to a thread milling cutter having an elongated shank, the tip of which comprises at least two cutting edges for forming a bore, which enclose together an end face angle and which are each separated by a chip groove, the cutter comprising at its outer side between the chip grooves respectively at least one toothed thread milling cutting edge provided with cutting studs.
2. Description of the Prior Art
Such a thread milling cutter is known from U.S. Pat. No. 4,651,374 and is formed like a drill having a drill tip formed by two cutting edges and four chip flutes or grooves extending spirally at the outer surface of the tool, two of which originate from the peripheral edges of the cutting edges and the other two of which terminate axially offset at the rear. In contrast to a drill, however, the spiral peripheral regions of the tool separated by the chip grooves are provided with tooth-like cutting lands or studs having clearance angles (undercut) produced by grinding and extending substantially in the peripheral direction of the tool.
The two drill cutting edges at the tip of the tool are inclined to each other at an angle measured from the side of the tool, which may, for example, be 120.degree.; the two cutting edges are generatrices of a cone face.
The end of the known thread milling cutter remote from the drill tip preferably comprises an increased diameter and is designed for clamping in a machine tool.
The purpose of the known thread milling cutter is to produce in the simplest and most economical manner a thread blind hole. For this purpose the thread milling cutter is first set in rotation, like a drill, about its longitudinal axis and sunk into the material to be provided with the thread blind hole until the desired drill hole depth is reached.
The tool is then slightly raised so that its tip is no longer centered by the conically deepened drill hole bottom and is set in such a rotation that the peripheral speed corresponds to the necessary cutting speed at the cutting lands or studs. The tool is now laterally offset with respect to the drill hole axis by the depth of the thread flight to be formed, always maintaining parallel alignment with said axis, whereupon the tool follows with its centre axis a circular curve having a radius corresponding to said lateral offsetting. In this manner the tool covers a path of at least 360.degree.. At the same time the tool is raised uniformly by the magnitude of the thread pitch. A finished blind hole thread is now milled over the entire drill hole length. The milling cutter now need only be centered again with respect to the drill hole and can then be withdrawn from the drill hole.
In this manner it has already proved possible by means of a special thread milling machine to drill in aluminum with a single spindle and thus a single thread milling cutter more than 120 blind holes per minute and provide them with an internal thread. The thread entry of said blind holes, however, inevitably has a burr which, on turning in threaded bolts, can get between the external and internal thread, impairing turning in of the threaded bolt and leading to damage of the thread. Burr removing is, however, not satisfactorily possible by machine and is usually done by hand and this partially eliminates the advantage of rapid drilling and thread cutting achieved with the known milling cutter.
The thread blind holes made in this manner also have at the drill hole bottom the usual drill termination extending beyond the lowest thread flight, so that the blind hole itself is deeper than the screw-in depth which can be achieved. It would, however, be advantageous in many cases if the thread flights or turns extended up to the drill hole bottom. However, making such a thread blind hole is exceedingly complicated; it must be carried out with specially ground taps and is therefore avoided whenever possible so that such a threaded blind hole is considered to be a design error in studies made during engineering training unless specific circumstances make such a blind hole design absolutely essential.
A series production of such blind holes is inconveivable, even more so with the afore-mentioned production rate.
A further disadvantage of the known thread milling cutters resides in that a separate tool is required for practially each thread nominal diameter. This requires, however, precisely in the case of precision thread drillings with relatively large diameter the availability of a large number of expensive thread milling cutters.